Title: Antisense therapy for DMD-Optimization and toxicology of AON for exon 45 skipping. Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy affecting 1 in every 3500 live male births. The disease is characterized by severe muscle wasting and weakness, which becomes clinically evident between the ages of 3 to 5 years. DMD is caused by so called frame-shift mutations in the dystrophin gene leading to premature termination with no protein synthesis whereas BMD is caused by mutations which create truncated, but in-frame transcripts with proteins of partial or nearly full function. Antisense oligonucleotide (AON)-mediated exon splicing (antisense therapy) has been shown to be effective for restoration of the open reading frame disrupted by mutations that cause DMD. The project P.I. has established the facts that near normal levels of dystrophin can be restored and maintained in the body-wide skeletal muscles through regular administration of a peptide-tagged morpholino AON (PPMO) in animal models with tolerable toxicity. We have also addressed several important issues critical for successful applications of the therapy to clinics. The development of a reliable in vitro system for AON selection enables us to establish highly effective AONs as specific drugs to target individual exons. We have defined the regimen of administration so therapeutic effect can be achieved with minimum toxicity for clinical trial. In this project/we aim to translate the successful experimental therapy to clinical treatment of DMD. The experimental plan is centered to achieve well-defined milestones: 1) Select most efficacious AON for targeting human dystrophin exon 45, the removal of which can treat a large proportion of DMD patients. This aim will be achieved with eatablished cell culture systems. 2) Determine the efficacy of selected PPMOs in animal models ahd off-target effects of the selected PPMOs in cell cultures. 3) Conduct toxicology tests, thus an investigational new drug application can be submitted to the FDA. Antisense therapy is effective in treating animal models of DMD and provides a realistic hope for treating the majority of DMD. We aim to translate the therapy into clinical trials.